1. Field of the Invention
The present invention relates to an insert casting structure in which a cylinder liner made of cast iron and having multiple projections on the outer circumferential surface is inserted within aluminum alloy through insert casting, and relates in particular to an insert casting structure possessing good thermal conductivity and bonding strength.
2. Description of the Related Art
Cylinder liners of cast iron are often mounted in a cylinder block of aluminum alloy in order to make the automobile engine compact and light weight. The following methods are known in the conventional art for manufacturing this type of cylinder block with cylinder liners. A cylinder liner is first set beforehand into the mold for casting the cylinder block. Casting material (aluminum alloy) is then poured into the mold. The outer circumference of the cylinder liner is enclosed by the aluminum alloy. In this case, improving the thermal conductivity of the insert casting structure where the cylinder liner and the cylinder block are bonded into a single piece is effective in improving engine performance.
Technology in the prior art for cylinder liners for insert casting is disclosed in Japanese Non-examined Patent Publication No. 2005-194983, U.S. Patent document No. 7,383,805 and in Japanese Non-examined Patent Publication No. 2004-209507. The technology disclosed in Japanese Non-examined Patent Publication No. 2005-194983 and U.S. Patent document No. 7,383,805 proposes improving the adherence and the bonding strength between the cylinder liner and the cylinder block by specifying fixed values for the height of projections, the number of projections and the projection area ratio on the outer circumferential surface of the cylinder liner. The technology disclosed in Japanese Non-examined Patent Publication No. 2004-209507 proposes improving the adherence and the thermal conductivity between the cylinder liner and the cylinder block, and making a thinner wall structure by using specific values for the arithmetic average roughness and developed length ratio of the outer circumferential surface of the cylinder liner. Moreover, thermal spraying of high thermal conductivity material such as aluminum alloy onto the casting surface of the cylinder liner is known as the conventional art for improving the thermal conductivity of the insert casting structure formed by inserting the cylinder liner within the aluminum alloy through insert casting.
The vicinity of the top dead center of the cylinder bore is subject to harsh thermal conditions in recent years due to engines with higher output and low fuel consumption. Moreover, the wall thickness between the cylinder bores must be made thinner to achieve a compact and low friction engine. The thermal conducting characteristics of the insert casting structure in which the cylinder liner is inserted within aluminum alloy through insert casting must be improved as a measure to reduce thermal effects. Making the wall thickness between the cylinder bores to thinner dimensions also requires that the cylinder liner be made thinner.
The technology disclosed in Japanese Non-examined Patent Publication No. 2005-194983 and U.S. Patent document No. 7,383,805 achieves high bonding strength but the thermal conductivity is inadequate. Moreover, maintaining the texture of the sliding surface and making the cylinder liner to thinner dimensions becomes difficult when the projection height is high. The technology disclosed in Japanese Non-examined Patent Publication No. 2004-209507 does not possess sufficient bonding strength between the outer circumferential surface of the cylinder liner and the aluminum alloy. The method of thermal spraying of high thermal conductivity material such as aluminum alloy onto the outer circumferential surface of the cylinder liner is high in terms of costs.